The present invention relates to a medium voltage load switch for medium voltage power lines and, more particularly, to an improved mechanism for opening and closing the contacts of a vacuum insulated switch.
Medium voltage load switches are generally rated 60 to 600 amperes and operate at voltages of 15 through 38 KV. They are used to connect or disconnect capacitor banks, transformers and other loads and devices commonly used in power distribution. Medium voltage load switches presently on the market use different schemes to open and close their contacts in different types of insulating media such as air, oil, vacuum and sulfur hexaflouride. Typically, such a switch includes a covered tank filled with an insulating medium. A pair of terminals extends through the lid of the switch into the tank. The terminals are surrounded by porcelain bushings. A plurality of contacts is disposed within the insulating medium. At least one of the contacts is movable and at least one of the contacts is stationary. The terminals are electrically coupled to the contacts such that when the contacts are closed, electric current may flow from one terminal to the other. Conversely, when the contacts are open, the flow of current is interrupted. Thus, the switch may be used to cut power at a given location along a medium voltage power line to allow servicing of the equipment downstream from the switch. The insulating medium in such a switch performs two functions: It prevents restrikes during contact opening and closing and it suppresses arcing between the electrically energized parts and the grounded tank and cover.
Two types of contact systems for medium voltage load switches are known in the industry: Contact bar and vacuum bottle. A typical contact bar switch includes two pairs of stationary contacts and one movable contact bar submerged in an insulating oil. The contacts are fixed to an electrically nonconductive support. Each terminal is coupled to one of the stationary contacts. The contact bar is pivotally connected to the support on which the stationary contacts are mounted. In one prior art device, a rigid rod is pivotally connected at one end to the contact bar and at the other end to a lever. The lever is in turn pivotally secured about a pin. A cam is also secured about the same pin. The cam may be connected to an over-center spring assembly. The contact bar may be moved into contact with the stationary contacts by manually or automatically rotating the pin to which the cam and lever are secured. As the pin rotates, the cam rotates and compresses the spring of the over-center spring assembly. After a certain degree of compression of the spring has occurred, the energy stored in the spring assembly is such that it pivots rapidly, thereby causing the cam to rotate swiftly and strike the lever. When the cam strikes the lever, the lever pivots about the pin, thereby causing movement of the rigid rod. As the rod moves, the contact bar pivots into electrical contact with the stationary contacts. In this manner, electrical contact is made from one terminal to the other.
In the vacuum bottle type switch, a vacuum bottle having one movable contact and one stationary contact which are enclosed within a sealed vacuum chamber is placed in a porcelain bushing. Again, one terminal is electrically coupled to the movable contact and the other terminal is electrically coupled to the stationary contact. In one prior art embodiment of a vacuum bottle switch, the movable contact is coupled to a series of toggle links. One of the links is coupled to a push/pull type solenoid. Current is applied to the solenoid to move the toggle links, thereby opening or closing the contacts.
In both the contact bar and vacuum bottle switches, the contacts must be opened and closed rapidly to prevent restrikes of the current. In the contact bar switch, the insulating medium prevents restriking between the contacts as they are opened and closed, and further prevents arcing between the contacts and the tank. Vacuum around the contacts prevents restriking in the vacuum bottle switch, however, no oil is required in the tank to prevent arcing since the operating mechanism in the tank is at low voltage.
Although the foregoing prior art devices are generally suitable for their intended purposes, they have several drawbacks. First, with respect to the vacuum bottle system described above, the solenoid directly drives the toggle link system for opening and closing the contacts. Thus, the vacuum bottle system may be manually opened but cannot be manually closed. Although the contact bar switch may be operated either by motor or manually, such a switch has a much shorter life than a vacuum bottle type switch.
Accordingly, it is an object of the present invention to provide an improved medium voltage load switch.
Another object of the present invention is the provision of an improved mechanism for opening and closing the contacts of a medium voltage load switch.
Yet another object of the present invention is the provision of a medium voltage load switch which may be opened and closed manually or electrically.
Yet another object of the present invention is to increase the mechanical life of a vacuum bottle switch and insulate the electrically energized parts from the tank and cover.
These and other objects of the present invention are attained by a mechanism for opening and closing the contacts of a vacuum switch including a drive link coupled to a movable contact of the vacuum bottle. A first toggle link is coupled to the drive link and a drop link is coupled to the toggle link and to a lever. A cam is coupled to the lever for imparting motion to the lever. A handle is coupled to the cam for rotating the cam. The cam is also coupled to a spring assembly. The spring assembly resists movement of the movable contact until a predetermined level of force is stored in the spring assembly.
According to another embodiment of the present invention, a latch is provided for holding the contacts in the open position. The latch may abut the drop link, a pin on which the drop link rotates or rollers disposed abut such a pin, when the contacts are in the open position.
According to another embodiment of the present invention, a second toggle link is coupled to the first toggle link and to the drop link.
According to another embodiment of the present invention, a third toggle link is coupled to the drive, first, second and drop links.
According to yet another embodiment of the present invention, a mounting tower is disposed in the switch. The second toggle link and the drive link are coupled to the mounting tower.
In another embodiment of the present invention, a torsion spring is coupled to the drive link and an over travel spring is disposed above the movable contact.
In still another embodiment of the present invention, the spring assembly comprises a coil spring, a shaft having a first end and a second end extending through the spring, a first head fixed to the first end of the shaft and a second head having a bore therein for slidably receiving the second end of the shaft. The first head is pivotally secured to the cam and the second head is pivotally secured to a cover of the switch.
Other objects, advantages and novel features of the present invention will become apparent when considering the following detailed description of the preferred embodiments in conjunction with the accompanying drawings.